Many optical networks comprise Wavelength Selective Switches (WSS) interconnected by Optical Multiplex Sections (OMS), each comprising a plurality of fibre optic spans. In such networks, signals of different wavelengths, known as channels, are multiplexed together and transmitted through the same OMS. The fibre optic spans making up the section of an OMS will typically be separated by optical amplifiers which will act as transponders, thereby receiving a degraded signal at the end of a first span, and transmitting a stronger signal at the start of the next span.
This process of re-amplification, whilst prolonging the OMS length, does itself introduce some errors into the signal. Moreover, the amount and impact that these errors will have will vary depending on the bandwidth and wavelength of each channel. There are several causes of per-channel power errors in a system related to gain mode amplifiers. Such factors include gain errors, tilt and ripple. In order to mitigate the effects of these errors, several methods have been developed to optimise the power level of signal input into an OMS.
One such method is Automatic Pre-emphasis Adjustment (APA), which is a power optimization technique which works on a link basis from a transmission point to a reception point. APA typically consists of monitoring the Bit Error Rate (BER) at a receiver and varying the per-channel pre-emphasis applied to the transmitter. This closed loop algorithm aims to tune the per-channel pre-emphasis in such a way as to optimise the resulting BER at the receiver end. One disadvantage of the APA technique is that it is suitable only for point-to-point routes, in that it requires the BER information to be fed back to the transmission side for each route in the network. Accordingly, using APA, the power algorithms required to calculate the per-channel pre-emphasis in a meshed network would become prohibitively complex.
A method which is compatible with meshed networks is enhanced Automatic Power Equalization (APE), which consists of equalizing the power of all channels (i.e. flat launch) at the beginning of each section, and enhancing the flat launch with a pre-emphasis profile in order to reduce gain errors. Currently, the required pre-emphasis profile is typically estimated using measurements taken by an Optical Channel Monitor (OCM), which can only provide per-channel power information and is therefore a much less reliable quality of transmission (QoT) indicator than the BER. Proposals put forth in the prior art relating to enhanced APE teach setting the pre-emphasis power to half of the measured per-channel power deviation at the end of the Optical Multiplex Section. This estimation does not take into account any disparities between the output powers of the various amplifiers in the OMS, nor does it take into account any differences between the span losses in the OMS.
Accordingly, when these differences are particularly acute (i.e. in the case of an OMS with large span and amplifier power heterogeneity), the above pre-emphasis estimation will significantly diverge from the optimal pre-emphasis profile.
There is therefore a need for an improved method of determining the per-channel power pre-emphasis needed in an OMS.